Here's what my dad's friend wrote me back, hopefully there's some useful info:
There are lots of things that can be causing the swelling. And without knowing the horses age or other contributing factors, it's kind of hard to say. One scenario with malnourished horses is that some horses, when starved and malnourished can contact Cushings Disease which causes swelling in the legs as well. These horses, for some reason develop longer shaggy hair, muscle weakness and can have pain in the hooves and leg areas. But that's only one scenario.
If he is not experiencing any pain in the hooves, that's a good sign (means it probably not founder, which usually occurs in the front hooves as well as the rear, founder is usually contributed to the horse over-eating grain or sweet feeds).
Another thought is, since the swelling is located on his rear legs and not the front, the swelling may be similar to water weight gain, Though this can be caused by a number of things, one thought is that the diet he is on now, is too "hot." If he was not used to eating a proper diet, too much of a good thing can actually be bad for him. Although the vet recommended the foal mixture, he may be getting too much of a dose of the protein and vitamins too soon, it should be more of a gradual build up. Depending on age, it can affect horses differently. With older horses, if they try to gain weight too fast, they can develop problems with their kidneys, lungs and heart, which in turn can cause swelling and water weight gain in the leg areas, usually knees and lower (in some cases, it can also cause heart failure).
Mainly bring them along slowly. Naturally hay (not alfalfa) and pasture are necessary, but you can also begin using a feed like Strategy (has protein and all, but isn't as "hot" as the sweet feed and alfalfa). After he shows signs of improvement, you could try switching to a small amount of oats twice each day for his grain (maybe 1/2 lb. each feeding and increase gradually over a couple of months), this along with the pasture and hay (not alfalfa) should keep him a happy camper.
Although he may too weak for exercising, you could but a halter on him and lead him around, even if he only walks, it can do him some good (mentally as well as physically).
I think it's great you rescue horses. Though I have a couple horses of my own, I am still learning about them as well, and with a mare in foal, guess I'll be learning a little more. Hope all works out well for you and Rex.
